Generally, a refrigerant-circulating cycle when a heat pump is used for heating has a closed loop formed by a compressor adapted to compress a refrigerant to high temperature and high pressure, a condenser adapted to condense the high temperature and high pressure refrigerant discharged from the compressor to a liquid phase by radiation at the indoor, expansion valves adapted to expand the liquid-phase refrigerant discharged from the condenser to a low pressure by means of a throttling action, and an evaporator adapted to evaporate the throttled refrigerant to a gaseous phase by means of the heat absorption at the outdoor.
Further, as is well known, the heat pump can be used for cooling when the refrigerant-circulating cycle is reversely operated, and therefore, the heat pump as a single device using a four-way valve, can selectively perform cooling and heating operations, thereby effectively utilizing a restricted space. Recently, thus, the heat pump becomes very popular in this field.
According to the heat pump, however, the surface temperature of an exterior heat exchanger serving as an evaporator during the heating operation in winter seasons is set lower than a dew-point temperature of outdoor air, such that frost is generated on the surface of the exterior heat exchanger. If the frost is accumulated, air-flowing is not good to cause the heat-exchanging between the outdoor air and the refrigerant to be made badly, thereby deteriorating the performance of the heat pump.
Moreover, as the specific volume of the refrigerant absorbed at the compressor based upon the decrease of an evaporation pressure becomes large, the compression efficiency becomes low and the discharge temperature is excessively increased, thereby causing the damage on the compressor.
To prevent such problems, therefore, a defrosting operation should be conducted under a given condition or for a given time. Thus, a hot gas bypass defrosting operation has been presented in conventional practices.
FIG. 1 shows the conventional heat pump using the hot gas bypass defrosting operation (as disclosed in Korean Utility Model Registration No. 20-0284796), and an explanation of the schematic configuration of the heat pump will be given below.
As shown, a discharge line of a compressor 11 is connected to an interior heat exchanger 12 as a condenser via a four-way valve 21, and an outlet of the condenser 12 from which the refrigerant is discharged is connected to an exterior heat exchanger 13. An outlet of the exterior heat exchanger 13 is connected to an inlet of the compressor 11 to which the refrigerant is supplied.
Between the interior heat exchanger 12 and the exterior heat exchanger 13 is provided an expansion valve 4 that is adapted to expand the liquid-phase refrigerant of high temperature and high pressure discharged from the interior heat exchanger 12 to a low pressure by means of a throttling action, so as to make the refrigerant easily evaporated, and a liquid receiver 43 is disposed at an inlet of the expansion valve 4, for supplying only the liquid-phase refrigerant to the expansion valve 4.
So as to conduct the defrosting operation, a bypass pipe 31 is connected at one end thereof between the output of the compressor 11 and the four-way valve 21 and is connected at the other end thereof between the exterior heat exchanger 13 and the expansion valve 4, while being controlled by means of a hot gas control valve 3. Further, a control valve 1 is disposed between the four-way valve 21 and the interior heat exchanger 12, and a control valve 2 is disposed between the liquid receiver 43 and the expansion valve 4, the control valves 1 and 2 serving as a structure for opening and closing the refrigerant pipe.
Referring to the defrosting operation of the cycle as mentioned above, if the defrosting operation is conducted for a given period of time at a state where the control valves 1 and 2 at the interior heat exchanger 12 are closed and the hot gas control valve 3 is opened, the high-temperature and high-pressure hot gas is introduced to the exterior heat exchanger 13 to cause the temperature at the exterior heat exchanger 13 to become raised, such that the frost or ice generated on the outside of the exterior heat exchanger 13 becomes removed. After completing the defrosting operation, a normal operation starts at a state where the control valves 1 and 2 are opened and the hot gas control valve 3 is closed, thereby returning to a normal heat pump cycle.
By the way, the hot gas bypass defrosting cycle of the conventional heat pump has had the following problems.
First, according to the conventional heat pump having the hot gas bypass defrosting cycle, the liquid-phase refrigerant that is not completely evaporated remain somewhat in the interior of the exterior heat exchanger 13, that is, at the inside of the evaporator, during the heating operation, such that they are accumulated in the lower tubes of the evaporator by its weight up to about 20% of the volume of the evaporator tube.
According to the conventional heat pump having the hot gas bypass defrosting cycle, moreover, the hot gas is introduced to the evaporator by using a single pipe, and in this case, even though the hot gas discharged from the compressor is bypassed up to a quantity of 100% to the evaporator, the liquid-phase refrigerant that is accumulated in the lower tubes of the evaporator are a little evaporated only on the top portion contacted with the hot gas, such that the refrigerant accumulated at the lower side that is not in contact with the hot gas still remain at the liquid phase. As a result, the hot gas is heat-exchanged with the refrigerant accumulated only on a portion of the evaporator tubes and is then circulated again to the compressor.
In general cases, during the defrosting operation the hot gas that is circulated again to the compressor 11 from the evaporator 13 is sufficiently heat-exchanged with the refrigerant remaining in the evaporator 13, such that it should be lowered at its temperature and pressure.
As mentioned above, however, since the high-temperature and high-pressure hot gas that has been bypassed up to a quantity of 100% to the evaporator is heat-exchanged with the refrigerant accumulated only on a portion of the evaporator tubes, the heat-exchanging operation is not completely conducted, thereby undesirably preventing the temperature and pressure of the hot gas from being sufficiently decreased.
The hot gas that is circulated again from the evaporator to the compressor exceeds an appropriate pressure, and thus, if it is recompressed by means of the compressor 11, an excessively high pressure is generated to apply an impact to the compressor, thereby making the compressor malfunctioned.
Therefore, according to the conventional heat pump having the hot gas bypass defrosting cycle, theoretically, the high-temperature and high-pressure hot gas is bypassed up to a quantity of 100% to the evaporator, but actually, the hot gas is bypassed up to only a quantity in a range between 20% and 30% to the evaporator when considering its stable operation, which of course accompanies a defect that the defrosting efficiency is substantially decreased.
Second, since the hot gas is bypassed up to only a quantity in a range between 20% and 30% to the evaporator as mentioned above, the conventional heat pump has a low defrosting efficiency. So as to achieve a successful defrosting operation, thus, the defrosting operation should be conducted for a relatively long period of time.
In the conventional heat pump, generally, the successful defrosting operation is conducted for 5-10 minutes or more, which is dependant upon the quantity of the accumulated frost. During the defrosting operation, the heating operation stops, which causes another problems that the indoor temperature becomes substantially low to an appropriate value and thus the heating operation inevitably starts again to maintain the appropriate indoor temperature at a state where the defrosting operation is not completely finished.
Therefore, the liquid-phase refrigerant that is accumulated in the lower tubes of the exterior heat exchanger 13 are not completely evaporated, and thus, the frost or ice generated on the outer surface of the lower tubes still remains thereon by a given quantity, while not fully removed therefrom.
If the incomplete defrosting operation is repeatedly conducted at the state where the frost still remains in the end of the lower tubes of the exterior heat exchanger 13, the frost becomes accumulated. As a result, the accumulated frost undesirably serves to block the tubes of the exterior heat exchanger 13, which closes the air-flowing passageway, thereby causing a state where heating is impossible.
Third, in the conventional heat pump having the hot gas bypass cycle as mentioned above, at the state where the liquid-phase refrigerant is kept accumulated in the lower tubes of the exterior heat exchanger 13, a difference of the quantity of a refrigerant is generated between the exterior heat exchanger 13 and the interior heat exchanger 12. At this state, if the defrosting operation is finished to return to the heating operation, the refrigerant in the exterior heat exchanger 13 flows at the liquid phase into the compressor 11, and therefore, the liquid compression occurs in the compressor 11, thereby making the compressor 11 easily have troubles.